(1) Field of the Invention
The present invention relates generally to an electrophotography, and more particularly to a method for printing a plurality of duplicated copies with the aid of the same and single electrostatic charge latent image once formed on an electrostatic charge retentive member such as a photoconductive or dielectric drum. Such a multiple duplication method generally comprises a step for forming an electrostatic charge latent image corresponding to an image of a document to be duplicated on a charge retentive member; a step for developing the latent image with toner particles to form a visible toner image on the charge retentive member; a step for transferring the developed toner image onto an image receiving member such as a sheet of paper; a step for fixing the transferred toner image on the image receiving member, and a step for repeating successively the developing, transferring and fixing steps for successively supplied image receiving members, while maintaining the latent image once formed on the charge retentive member.
(2) Description of the Prior Art
Various methods have been proposed for printing a plurality of duplicated copies in electrophotography. For instance, U.S. Pat. No. 2,951,443 describes such a method. However, it has been found that this method produces copies having smaller picture or print density in accordance with the number of copies. U.S. Pat. No. 3,363,555 discloses a multiple duplication method in which a transferring bias voltage applied to a transferring roller is changed in accordance with the number of copies to maintain a constant print density. When the transferring bias voltage is high, an imagewise bright background of the charge latent image might be electrified and thus so-called fog or overdevelopment might occur. This fog is easily produced under a high humidity. Japanese patent application laid-open Publication No. 89,421/76 (corresponding to U.S. patent application Ser. No. 536,712) also discloses a multiple duplication method wherein a developing bias voltage applied to a developing electrode is adjusted in accordance with the number of copies in order to make print density constant. This method does not provide copies having excellent image quality, because the density of higher print density part is developed to an insufficient extent, when the developing bias voltage is so adjusted that copies having constant print density at lower density part can be obtained, while the density of lower print density part is liable to be overdeveloped, when the developing bias voltage is adjusted to provide copies having constant print density at higher density part. Moreover this method also produces said fog in the imagewise bright background.